X-rays usually are produced in a vacuum, but for many purposes it is desirable to apply them in air. For soft X-rays, especially those having photon energies of less than about 5 keV, a problem arises in bringing the X-rays from the vacuum into air, because a window that is thick enough and strong enough to withstand the pressure difference between the vacuum and the air is opaque to the X-rays, except in very small windows. The problem is especially serious in X-ray lithography, where it is desirable to illuminate large areas.
The present invention provides simple, inexpensive, convenient means for overcoming the problem.
It is shown in U.S. Pat. No. 4,058,486, Nov. 15, 1977, of P. J. Mallozzi, H. M. Epstein, R. G. Jung, D. C. Applebaum, B. P. Fairand, and W. J. Gallagher, for Producing X-rays, that an intense point source of X-rays can be generated by focusing a laser beam onto a solid target. Neodymium laser light focused onto a solid slab target has been converted into X-rays with an efficiency greater than 25 percent, with several tens of joules of X-rays emanating from an essentially point source (about 100 microns diameter) in a nanosecond. The X-ray pattern produced with iron targets irradiated with about 100-joule laser pulses at a 45 degree angle of incidence is substantially omnidirectional. The conversion efficiency of greater than 25 percent refers to X-rays which are radiated away from the slab and pass perpendicularly through 3000 Angstroms of plastic (paraline) coated with 2000 Angstroms of aluminum. This conversion efficiency is thus a lower bound and refers only to the portion of the spectrum above about 300 electron volts. Most of the observed X-rays lie between about 0.3 and 1.5 keV, with a small but useful fraction having energies as high as 10 to 100 keV. In a densitometer tracing of a bent crystal spectrograph taken with a KAP crystal, the radiation appears to be mostly lines in the spectral interval of about 0.7 to 1.2 keV. The unusual sharpness of the spectral detail is due to the tiny dimensions of the source. This novel point source of X-rays provides a spectrum tuneable throughout a range of about 0.1 to 100 keV.
Apparatus according to the present invention typically employs X-ray producing means of the type described above. It may, however, use other somewhat similar means, such as equipment that uses an electron beam, rather than a laser beam, for producing the X-rays.